Ultrasonic welding is one of the techniques for joining two or more similar or dissimilar metals in the form of wires, ribbons, foils, or sheets. Recently, USW has been used to join battery cell terminals and interconnector busbars in battery modules of high-voltage (HV) battery packs for electric vehicles (EVs). An USW system comprises an electronic ultrasonic generator, an actuator, an anvil and an ultrasonic stack. The ultrasonic stack includes a converter which converts electrical signals to mechanical vibrations, a booster which modifies or maintains the amplitude of the mechanical vibrations, and a horn. During an USW process, the components to be joined are clamped between the horn and the anvil by their work surfaces. One of the components, normally the thickest one, is held stationary by the anvil, while the other component(s), usually the thinner one(s), is gripped and vibrated by the horn against the stationary component at an ultrasonic frequency of 20 to 40 kHz. The intense friction mingles the metals' atomic structures, forming thousands of solid-state atomic bonds in the weld zone, thus creating a joint.
The work surfaces of the horn and the anvil consist of patterns of protrusions or knurls. To date, the protrusions on the horn work surface(s) have been truncated pyramids. This type of work surface geometry has posed several problems. Prohibitively high energies were required for joining multiple layers of high-strength metal sheets/foils of various thicknesses and with higher-melting-temperature and/or harder coatings. Stress concentrations around the sharp edges of the truncated pyramids caused the excess thinning and/or fracture of the outermost layers in the joints made. In addition, the buildup of metal fines and the chipping of the truncated pyramids induced the process instability and shortened the life of USW tools, in particular, the life of the horn. Therefore, the design of the work surfaces of the horn and the anvil, particularly the horn, critically impacts the quality, durability and robustness of the USW joints, and influences the USW tool life and process reliability.
Accordingly, a radical tooling solution is desired to address the problems associated with the geometric design of the USW tools.